Summary of Significant Accounting Policies - Schedule of Allowance for Doubtful Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Allowance for Doubtful Accounts Receivable [Roll Forward]
Allowance at beginning of year	$ 23.0	$ 22.4	$ 18.3
Additions charged to expense	14.0	16.9	14.6
Net deductions	(12.1)	(16.3)	(10.5)
Allowance at end of year	24.9	23.0	22.4
Spire Missouri
Allowance for Doubtful Accounts Receivable [Roll Forward]
Allowance at beginning of year	14.9	16.0	14.1
Additions charged to expense	12.7	12.3	11.9
Net deductions	(9.5)	(13.4)	(10.0)
Allowance at end of year	18.1	14.9	16.0
Spire Alabama
Allowance for Doubtful Accounts Receivable [Roll Forward]
Allowance at beginning of year	6.3	3.9	2.6
Additions charged to expense	0.9	4.7	2.1
Net deductions	(1.7)	(2.3)	(0.8)
Allowance at end of year	$ 5.5	$ 6.3	$ 3.9